Cellulose degrading bacteria are present in considerable number in ceca of rats fed both low- and high-fiber diets. While the number of cellulolytic organisms is the same in both ceca, the kinds of organisms are apparently different. Hemicellulose digestion is also extensive in both ceca. Because of the obvious important role the cellulose degrading organisms play in the gut fermentation these organisms will be studied in depth (nutrition, physiology and metabolism). In addition to cellulolytics, other organisms have been isolated from 6 different experiments. These organisms are presently being characterized in order to determine whether the presence of "dietary fiber" can cause the microbial flora both to be different and to have the capability to ferment this fiber. From these results it may be possible to determine whether the gut bacteria play a direct role in the beneficial effects of fiber in the diet.